Semiconductor device and its manufacturing method

ABSTRACT

Nickel is selectively held in contact with a particular region of an amorphous silicon film. Crystal growth parallel with a substrate is effected by performing a heat treatment. A thermal oxidation film is formed on the silicon film by performing a heat treatment in an oxidizing atmosphere containing a halogen element. During this step, in the silicon film, impurities included such as oxygen or chlorine, are segregated with extending along the crystal growth, the crystallinity is improved, and the gettering of nickel element proceeds. A thin-film transistor is formed so that the direction connecting source and drain regions coincides with the above crystal growth direction. As a result, a TFT having superior characteristics such as a mobility larger than 200 cm 2 /Vs and an S value smaller than 100 mV/dec. can be obtained.

This application is a Divisional of application Ser. No. 08/785,536,filed Jan. 17, 1997 U.S. Pat. No. 6,093,934.

BACKGROUND OF THE INVENTIONS

1. Field of the Invention

The present invention relates to a crystalline thin-film semiconductorand a manufacturing method thereof. The invention also relates to asemiconductor device using the above thin-film semiconductor and amanufacturing method thereof.

2. Description of Related Art

Techniques are known in which a crystalline silicon film is formed on aglass or quartz substrate and thin-film transistors (hereinafterreferred to as TFTs) are formed by using the silicon film. Such TFTs arecalled high-temperature polysilicon TFTs or low-temperature polysiliconTFTs.

In the case of high-temperature polysilicon TFTs, a crystalline siliconfilm is formed by a technique including a heat treatment at a relativelyhigh temperature of 800-900° C. It can be said that this technique isderived from an IC manufacturing process using a single crystal siliconwafer. Naturally, high-temperature polysilicon TFTs are formed on aquartz substrate, which withstand the above-mentioned high temperature.

On the other hand, low-temperature polysilicon TFTs are formed on aglass substrate, which is inexpensive but is apparently lower in heatresistance than a quartz substrate. To form a crystalline silicon filmfor low-temperature polysilicon TFTs, heating at lower than 600° C.which a glass substrate can withstand or laser annealing which causesalmost no thermal damage on a glass substrate is performed.

The high-temperature polysilicon TFT is advantageous in that TFTs havinguniform characteristics can be integrated on a substrate.

On the other hand, the low-temperature polysilicon TFT is advantageousin that a glass substrate can be used which is inexpensive and caneasily be increased in size.

According to the current manufacturing techniques, there are no largedifferences in characteristics between the high-temperat ure polysiliconTFT and the low-temperature polysilicon TFT. That is, in both cases, themobility is 50-100 cm²/Vs and the S value is 200-400 mV/dec. (V_(D)=1V).

However, these values are much worse than those of MOS transistorsformed on a single crystal silicon wafer. In general, the S value of MOStransistors formed on a single crystal silicon wafer is 60-70 mV/dec.

At present, there are active matrix liquid crystal display devices inwhich an active matrix circuit and peripheral driver circuits areintegrated on the same substrate by using TFTs. In this type ofconfiguration, the source driver circuit of the peripheral drivercircuits is required to operate at a frequency higher than a little morethan 10 MHz. However, at present, a circuit using high-temperaturepolysilicon TFTs or low-temperature polysilicon TFTs can provide amargin of operation speed that is as small as several megahertz.

For this reason, at present, a liquid crystal display device isconstituted by dividing its operation (called “divisional driving”).However, this method has several problems; for example, stripes appearon the screen due to, for instance, a slight deviation in the divisiontiming.

SUMMARY OF THE INVENTION

It is now considered a configuration in which not only peripheral drivercircuits (constituted of a shift register circuit and a buffer circuit)but also an oscillation circuit, a D/A converter, an AD converter, anddigital circuits for various kinds of image processing are integrated onthe same substrate.

However, the above-mentioned oscillation circuit, D/A converter, A/Dconverter, and digital circuits for various kinds of image processingare required to operate even at higher frequencies than the peripheraldriver circuits. Therefore, it is very difficult to constitute suchcircuits by using high-temperature polysilicon TFTs or low-temperaturepolysilicon TFTs as long as they are formed by the current manufacturingtechniques.

On the other hand, integrated circuits of MOS transistors formed on asingle crystal silicon wafer which circuits can operate at more than 100MHz have already been put to practical use.

An object of the present invention is to provide a TFT which canconstitute a circuit that is required to perform a high-speed operation(generally at more than tens of megahertz).

Another object of the invention is to provide a TFT whosecharacteristics are equivalent to those of a MOS transistor formed on asingle crystal silicon wafer. It is also intended to provide a means formanufacturing such a TFT. It is further intended to provide asemiconductor device having a required function by using TFTs having sosuperior characteristics.

According to one aspect of the invention, there is provided asemiconductor device using a thin-film transistor that uses, as anactive layer, a crystalline silicon film formed on a substrate having aninsulating surface, wherein the crystalline silicon film has a crystalstructure that is continuous in a predetermined direction, and grainboundaries extending in the predetermined direction; the predetermineddirection is at a predetermined angle with a direction connecting asource region and a drain region of the thin-film transistor; and animpurity is segregated in the grain boundaries.

FIGS. 6 and 7 show an example of a crystalline silicon film having theabove-mentioned crystal structure. FIGS. 6 and 7 are photographs ofobtained by observing the surface of a 250-Å-thick crystalline siliconfilm with a transmission electron microscope (TEM). FIG. 7 is anenlargement of part of the photograph of FIG. 6.

The crystalline silicon film of FIGS. 6 and 7 can be obtained by amanufacturing process of a first embodiment of the invention which willbe described later.

FIGS. 6 and 7 show a crystal structure that continuously extends fromthe bottom left to the top right in these drawings, as well as manygrain boundaries extending substantially parallel with the abovedirection.

As is apparent from the crystal structure shown in FIG. 7, thiscrystalline silicon film is a collection of many crystallizations(crystalline silicon grains) each having a crystal structure extendingin the particular direction. The width of the crystallizations is500-2,000 Å, or from about the thickness of the crystalline silicon filmto 2,000 Å.

Many definite grain boundaries are arranged, at intervals,perpendicularly or substantially perpendicularly (in the direction fromthe bottom right to the top left in these drawings) to the direction inwhich the crystal structure has continuity: the crystal structure isdiscontinuous (continuity is lost) in the former direction.

The continuity of the lattice structure is substantially maintained inthe direction in which the crystal structure has continuity. In thisdirection, the scattering and trapping of carriers during their movementoccur at a much smaller possibility than in the other directions.

That is, it can be considered that a substantial single crystal state,in which carriers are not scattered or are hardly scattered by grainboundaries, is established in the direction in which the crystalstructure has continuity.

Further, oxygen or halogen elements are confirmed being in segregatedstate in the grain boundaries. This is confirmed by analyzing X-rayspectra generated as irradiating with a converged electron beam. It isconsidered that the oxygen and halogen elements are segregated in thegrain boundaries at the time of forming a thermal oxide film aftercrystallization.

The above-mentioned aspect of the invention defines the relationshipbetween the direction in which the crystal structure has continuity andthe direction connecting the source and drain regions of a thin-filmtransistor. To attain a high-speed operation, it is desired that thedirection in which the crystal structure has continuity coincide orsubstantially coincide with the direction connecting the source anddrain regions. This provides a configuration in which carriers can movemost easily.

The characteristics of a thin-film transistor can be controlled bysetting the angle between the above two directions at a proper value.For example, in the case of forming a number of thin-film transistorgroups, the characteristics of a plurality of groups can be madedifferent from each other by changing the angle between the twodirections from one group to another.

A thin-film transistor in which the active layer is bent to assume anN-like or a square-bracket-like, or even an M-like shape, that is, theline connecting the source and drain regions is bent can be formed inthe following manner. That is. the direction in which the crystalstructure has continuity is so set as to coincide with the carriermoving direction (as a whole) in the channel region.

Also in this case, the fastest operation can be expected when the anglebetween the carrier moving direction and the direction in which thecrystal structure has continuity is set at 0°. It is apparent that thisangle may be set at a proper value other than 0°, when necessary.

According to another aspect of the invention, there is provided asemiconductor device using a thin-film transistor that uses, as anactive layer, a crystalline silicon film formed on a substrate having aninsulating surface, wherein the crystalline silicon film is anisotropicin a grain boundary extending direction; the predetermined direction isat a predetermined angle with a direction connecting a source region anda drain region of the thin-film transistor; and an impurity issegregated in the grain boundary.

According to still another aspect of the invention, there is provided asemiconductor device using a thin-film transistor that uses, as anactive layer, a crystalline silicon film formed on a substrate having aninsulating surface, wherein the crystalline silicon film is anisotropicin a grain boundary extending direction; the grain boundary extendingdirection and a carrier moving direction in a channel region of thethin-film transistor form a given angle; and an impurity is segregatedin the grain boundary.

To obtain the crystalline silicon film of the invention, it is necessaryto perform a heat treatment after introducing a metal element thataccelerates crystallization of silicon as typified by nickel into anamorphous silicon film, and further perform a heat treatment in anatmosphere containing a halogen element. The heat treatment to form athermal oxide film causes oxygen and halogen elements in the atmosphereto deposit into the grain boundary.

As the metal element, nickel is the best in terms of reproducibility andeffects. In general, the metal element may be one or a plurality ofelements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd,Os, Ir, Pt, Cu, and Au.

Where nickel is used, the concentration of nickel left in a finalsilicon film is 1×10¹⁴ to 5×10¹⁸ atoms/cm³. If the gettering conditionsof a thermal oxidation film are refined, the upper limit of theconcentration can be reduced to 5×10¹⁷ atoms/cm³. The concentration canbe measured by the SIMS (secondary ion mass spectrometry).

In general, the lower limit of the nickel concentration is 1×10¹⁶atoms/cm³. This is because when a balance with the cost is considered,it is usually difficult to eliminate the influences of nickel attachedto a substrate and an apparatus used.

Therefore, when an ordinary manufacturing process is employed, theconcentration of residual nickel is in a range of 1×10¹⁶ to 5'10¹⁷atoms/cm³.

Since metal element moves into a thermal oxidation film in the step offorming it, metal element has a concentration gradient or distributionin the thickness direction of a resulting crystalline silicon film.

It is generally observed that the concentration of the metal element inthe crystalline silicon film increases toward the boundary of thethermal oxidation film. Under certain conditions, it is observed thatthe concentration of the metal element increases toward a substrate oran undercoat film, i.e., toward the lower-side boundary.

Where a halogen element is contained in an atmosphere used in formingthe thermal oxidation film, the halogen element assumes a concentrationdistribution similar to that of the metal element. That is, theconcentration increases toward the upper surface and/or lower surface ofthe crystalline silicon film.

In the invention, it is preferred that the crystalline silicon filmfinally has a thickness of 100-750 Å. It is even preferred that thethickness be 150-450 Å. With the thickness in such ranges, the uniquecrystal structure in which the crystallinity is continuous in onedirection as shown in FIGS. 6 and 7 can be obtained with highreproducibility in a more enhanced manner.

It is necessary to determine the thickness of the final crystallinesilicon film by considering the fact that the thickness is decreased bythe formation of the thermal oxidation film.

According to a further aspect of the invention, there is provided amanufacturing method of a semiconductor device, comprising the steps offorming an amorphous silicon film on a substrate having an insulatingsubstrate; selectively introducing a metal element for acceleratingcrystallization of silicon into a portion of the amorphous silicon film;performing a heat treatment, to thereby effect crystal growth parallelwith the substrate in a direction from the portion in which the metalelement has been introduced to the other portions; performing a heattreatment at 800-1,100° C. for 30 minutes or more by an electricalfurnace in an oxidizing atmosphere containing a halogen element, to forma thermal oxidation film; removing the thermal oxidation film; arranginga direction connecting a source region and a drain region with a crystalgrowth direction substantially. and segregating an impurity along thecrystal growth direction.

The crystalline silicon film of the invention can be obtained by theabove manufacturing process. Further, a MOS thin-film transistorutilizing the uniqueness of its crystal structure can be formed.

A metal element can be introduced by various methods, among which are amethod of applying a solution containing the metal element, a methodusing CVD, a method using sputtering or evaporation, a plasma processingmethod using an electrode that contains the metal element, and a methodusing gas absorption.

To introduce a halogen element, there may be used a certain means forcausing an oxidizing atmosphere (for instance, an oxygen atmosphere) tocontain HCl, HF, HBr, Cl₂, F₂, Br₂, CF₄, or the like.

It is effective to introduce a hydrogen gas into an atmosphere used informing a thermal oxidation film, and thereby utilize the action of wetoxidation.

The temperature is a very important factor in forming a thermaloxidation film. To obtain a TFT that can operate in itself at more thanseveral tens of megahertz and has a small S value of less than 100mV/dec. (described later), it is preferable that the temperature of aheat treatment for forming a thermal oxidation film be more than 800° C.It is even preferable that the temperature be more than 900° C.

The lower limit depends upon a pressure at which the annealing isperformed and a vapor pressure of the halogen compound of the materialto be gettered. That is, when the vapor pressure of the halogen compoundis smaller than the pressure of the annealing atmosphere. the getteringefficiency is not so high. For example, the vapor pressure of nickelchloride is 38.9 mmHg at 541° C. and 820.6 mmHg at 994° C. Accordingly,when the annealing is performed at the atmospheric pressure (760 mmHg),the effect of the gettering is significantly increased when thetemperature is 994° C.

It is appropriate that the upper limit of the above temperature be setat about 1,100° C., which is the upper limit a quartz substratewithstands.

According to another aspect of the invention, there is provided amanufacturing method of a semiconductor device, comprising the steps offorming an amorphous silicon film on a substrate having an insulatingsubstrate; selectively introducing a metal element for acceleratingcrystallization of silicon into a portion of the amorphous silicon film;performing a heat treatment, to thereby effect crystal growth parallelwith the substrate in a direction from the portion in which the metalelement has been introduced to the other portions; performing a heattreatment at 800-1,100° C. for 30 minutes or more by an electricalfurnace in an oxidizing atmosphere containing a halogen element, to forma thermal oxidation film; removing the thermal oxidation film; andarranging a carrier moving direction in a channel region to coincidewith or substantially coincide with a crystal growth direction; andsegregating an impurity along the crystal growth direction.

The above manufacturing method pays attention to the carrier movingdirection in the channel region, and defines the relationship betweenthe carrier moving direction and the crystal growth direction (i.e., thedirection in which the crystal structure has continuity or grainboundaries extend).

This manufacturing method is effective even in a case where the lineconnecting the source and drain regions is bent.

A specific example of the invention will be described below. In thetechnique of forming a crystalline silicon film by crystallizing anamorphous silicon film by heating it, a heat treatment is performed in astate that nickel is held in contact with a portion of the surface ofthe amorphous silicon film, so that crystal growth proceeds parallelwith a substrate from the above portion to the other portions.

Subsequently, a thermal oxidation film is formed on the surface of aresulting crystalline silicon film by performing a heat treatment at800-1,100° C. for 30 minutes or more by an electrical furnace in anoxidizing atmosphere containing a halogen element.

The thermal oxidation film is then removed. A crystalline silicon filmthus obtained has a structure in which grain boundaries extend in aparticular direction as shown in FIGS. 6 and 7 and the crystal structureis continuous in the same direction.

A TFT having superior characteristics can be obtained by making thecarrier moving direction during an operation coincide with the directionof the continuous crystal growth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D and 2A-2E show a manufacturing process of a TFT according toa first embodiment of the present invention;

FIGS. 3A-3D show a manufacturing process of a TFT according to a secondembodiment of the invention;

FIGS. 4A-4D show a manufacturing process of a TFT according to a thirdembodiment of the invention;

FIGS. 5A-5E show a manufacturing process of a TFT according to a fourthembodiment of the invention;

FIGS. 6 and 7 are electron microscope photographs of a silicon thinfilm;

FIGS. 8A-8F schematically show various semiconductor devices using TFTs;and

FIGS. 9A and 9B schematically illustrate how crystal growth proceeds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

This embodiment is directed to a method of effecting crystal growthparallel with a substrate (called “lateral growth”) by selectivelyintroducing into an amorphous silicon film a metal element thataccelerates crystallization of silicon.

FIGS. 1A-1D and FIGS. 2A-2E show a manufacturing process according tothis embodiment. First, a 3,000-Å-thick silicon oxide film as anundercoat film 202 is formed on a quartz substrate 201. The undercoatfilm 202 can be omitted if the quartz substrate 201 is sufficientlysmooth and if sufficient cleaning is performed.

Although the use of a quartz substrate is preferable at present, theinvention is not limited to the use of a quartz substrate and othersubstrates can also be used as long as they withstand a heat treatmenttemperature.

Next, a 600-Å-thick amorphous silicon film 203 as a starting film of acrystalline silicon film is formed by low-pressure thermal CVD. It ispreferred that the amorphous silicon film 203 be thinner than 2,000 Å.

Thereafter, a 1,500-Å-thick silicon oxide film (not shown) is formed andthen patterned into a mask 204 which has an opening 205. The amorphoussilicon film 203 is exposed in the opening 205.

The opening 205 has a long and narrow rectangular shape in which thelongitudinal direction extends perpendicularly to the paper surface ofFIGS. 9A-9D. It is appropriate that the opening 205 be wider than 20 μmand have a necessary length in the longitudinal direction.

After a nickel acetate solution containing nickel (nickel element) at 10ppm in terms of weight is applied, and excessive part of the appliedsolution is removed by spin drying with the use of a spinner (notshown).

Thus, there is obtained a state in which nickel exists as indicated by abroken line 206 in FIG. 1A. In this state, nickel is so held as to bebrought in contact with part of the amorphous silicon film selectively,i.e., at the bottom of the opening 205.

A heat treatment is performed in this state at 640° C. for 4 hours in anitrogen atmosphere in which hydrogen is contained at 3% but the contentof oxygen is minimized. As a result, crystal growth proceeds parallelwith the substrate 201 as indicated by arrow 207 in FIG. 1B. FIG. 9B isa top view schematically showing how the crystal growth proceeds.

This crystal growth proceeds around from the opening 205 from whichnickel has been introduced. This type of crystal growth that is parallelwith the substrate is called lateral growth.

A crystalline silicon film obtained by the lateral growth has a muchsmoother surface than a conventional high-temperature or low-temperaturepolysilicon film. This is considered due to the fact that in the formercase grain boundaries extend approximately in the same direction.

General polysilicon films have surface asperities of more than ±100 Å.In contrast, it was observed that silicon films obtained by lateralgrowth according to this embodiment have asperities that are as small as±30 Å. Since the asperities deteriorate the characteristics of aboundary with a gate insulating film, they should be as small aspossible.

Under the above-mentioned heat treatment conditions for thecrystallization, the lateral growth can proceed over more than 100 μm.Thus, a silicon film 208 having a lateral growth region is formed.

The heat treatment for the above crystal growth may be performed at450-1,100° C., the upper limit being restricted by the heat resistanceof the substrate. To obtain a certain lateral growth length, it ispreferred that the heat treatment be performed at more than 600° C. Evenif the temperature is set higher than 600° C., no remarkable improvementis obtained in crystal growth length or crystallinity.

The silicon oxide mask 204 that was used to selectively introduce nickelis now removed. Thus, the state of FIG. 1C is obtained.

In this state, nickel does not exist uniformly. In particular, nickelexists at relatively high concentrations in the opening 205 and theleading portion of the crystal growth indicated by arrows 207.Therefore, in forming an active layer, it is important to avoid suchregions. That is, it is important that an active layer not include aregion where nickel obviously has a higher concentration than in theother regions.

To improve the crystallinity, laser light irradiation may be performedin the state of FIG. 2C. The laser light irradiation has an effect ofdisperse a cluster of nickel in the film 208, to thereby facilitatelater removal of nickel. The laser light irradiation at this stage doesnot cause any further lateral growth.

The laser light may be excimer laser light having a wavelength in theultraviolet range. For example, a KrF excimer laser (wavelength: 248 nm)or a XeCl excimer laser (wavelength: 308 nm) may be used.

Next, a heat treatment is performed at 950° C. for 30 minutes or more byan electrical furnace in an oxygen atmosphere containing HCl at 3 volume%, to form a 200-Å-thick thermal oxidation film 209. As a result of theformation of the thermal oxidation film 209, the thickness of thesilicon film 208 is reduced by about 100 Å to about 500 Å.

In the above step, silicon elements in an unstable bonding state in thefilm 208 are used to form the thermal oxidation film 209, so thatdefects in the film 208 are reduced in number and hence a higher degreeof crystallinity is obtained.

At the same time, due to the formation of the thermal oxidation film 209and the action of chlorine, nickel elements are gettered from the film208.

Naturally nickel is introduced into the thermal oxidation film 209 at acomparatively high concentration, and the nickel concentration of thesilicon film 208 becomes relatively small.

The thermal oxidation film 209 is then removed. Thus, the crystallinesilicon film 208 with a decreased nickel content remains. As shown inFIGS. 6 and 7, the crystalline silicon film thus obtained has a crystalstructure extending in one direction (i.e., in the crystal growthdirection). That is, a number of long and thin cylindricalcrystallizations extend parallel with each other in one direction withgrain boundaries separating the crystallizations. Oxygen and chlorineare segregated in the crystal boundaries which extends in one direction(i.e., in the crystal growth direction).

The crystal boundaries in which the impurities are segregated performenergy barriers. The energy barriers have a function of limitingcarriers to move along the crystal growth direction.

Subsequently, the crystalline silicon film 208 is patterned into apattern 210 of a lateral growth region. This island-like region 210 willbecome a TFT active layer.

The patterning is performed with an orientation that makes the directionconnecting a source region and a drain region at least substantiallycoincide with the crystal growth direction. With this orientation, thecarrier movement direction is made equal to the direction in whichcrystal lattices extend continuously, to enable provision of a TFThaving superior characteristics.

Thereafter, a 300-Å-thick thermal oxidation film 211 is formed on thepattern 210 by a heat treatment of 950° C. for 30 minutes or more by anelectrical furnace in an oxygen atmosphere containing HCl at 3%. In thisstate, the pattern (to become the active layer) 210 has a totalthickness of 350 Å (including the thickness of the thermal oxidationfilm 211). A sum of thicknesses of the thermal oxide films formed isthicker than the final thickness of the crystalline silicon film.

In the step of forming the thermal oxidation film 211, the same effectsas in forming the thermal oxidation film 209 can be obtained. Thethermal oxidation film 211 will become part of a gate insulating film ofthe TFT.

Next, a 1,000-Å-thick silicon oxide film 304 to constitute the gateinsulating film together with the thermal oxidation film 211 is formedby plasma CVD. (FIG. 2A)

A 4,000-Å-thick aluminum film to constitute a gate electrode is thenformed by sputtering. Scandium is included in the aluminum film at 0.2wt % to suppress occurrence of hillocks and whiskers in a later step.Hillocks and whiskers are needle or prickle-like protrusions as formedby abnormal growth of aluminum.

After the formation of the aluminum film, a dense anodic oxide film (notshown) is formed in the following manner. The electrolyte is an ethyleneglycol solution containing tartaric acid at 3%. The aluminum film isused as the anode while platinum is used as the cathode. In this step, a100-Å-thick, dense anodic oxide film is formed on the aluminum film.

The anodic oxide film (not shown) has a role of improving the adhesionof a resist mask that will be formed later. The thickness of the anodicoxide film can be controlled by the application voltage during theanodization.

After a resist mask 306 is formed, the aluminum film is patterned into apattern 305. Thus, the state of FIG. 2B is obtained.

Anodization is again performed in this state in an electrolyte which isa 3% aqueous solution of oxalic acid. The aluminum pattern 305 is usedas the anode. As a result, a porous anodic oxide film 308 is formed.Since the resist mask 306 is in close contact with the top face of thealuminum pattern 305, the anodic oxide film 308 is selectively formed onthe side face of the aluminum pattern 305.

The thickness of the anodic oxide film 308 can be increased to severalmicrometers. In this embodiment, the thickness is set at 6,000 Å. Thegrowth length (i.e., thickness) of the anodic oxide film 308 can becontrolled by the anodization time.

After the resist mask 306 is removed, a dense anodic oxide film is againformed. That is, anodization is again performed by using theabove-mentioned electrolyte, i.e., an ethylene glycol solutioncontaining tartaric acid at 3%.

In this step, a dense anodic oxide film 309 is formed as shown in FIG.2C because the electrolyte enters into the porous anodic oxide film 308.

The thickness of the dense anodic oxide film 309 is set at 1,000 Å. Thethickness is controlled by the application voltage.

Next, the exposed portions of the silicon oxide film 304 as well asparts of the thermal oxidation film 300 are etched by dry etching. Theporous anodic oxide film 308 is then removed by using a mixed acid ofacetic acid, nitric acid, and phosphoric acid. Thus, the state of FIG.2D is obtained.

Impurity ions are implanted in this state. In this embodiment, P(phosphorus) ions are implanted by plasma doping to form an n-channelTFT.

In this step, heavily doped regions 311 and 315 and lightly dopedregions 312 and 314 are formed. This is because the residual siliconoxide film 310, which serves as a semi-transparent mask, shields part ofions launched.

The regions doped with impurity ions are then activated by illuminatingthose with laser light (or strong light emitted from a lamp). In thismanner, a source region 311, a channel forming region 313, a drainregion 315, and low-concentration impurity regions 312 and 314 areformed in a self-aligned manner. The region 314 is called a LDD (lightlydoped drain) region. (FIG. 2D).

If the dense anodic oxide film is made thicker than 2,000 Å, offset gateregions as thick as the dense anodic oxide film can be formed outsidethe channel forming region 313.

Although offset gate regions are formed even in this embodiment, theyhave only little contribution because of their small dimension, and theyare not shown in the drawings to prevent the drawings from becomingunduly complex.

To make the dense anodic oxide film 309 thicker than 2,000 Å, anapplication voltage of more than 200 V is needed. In this case,sufficient case should be taken for reproducibility and safety.

Next, a silicon oxide film, a silicon nitride film, or a laminate filmthereof is formed as an interlayer insulating film 316. Alternatively,the interlayer insulating film 316 may be composed of a silicon oxidefilm or a silicon nitride film and a resin film formed thereon.

After contact holes are formed through the interlayer insulating film316, a source electrode 317 and a drain electrode 318 are formed. Thus,a TFT is completed as shown in FIG. 3E.

The TFT of this embodiment exhibits much superior characteristics thatwere not obtained conventionally. For example, an NTFT (n-channel TFT)has a mobility of 200-300 cm²/V.s and an S-value of 75-90 mV/dec.(V_(D)=1 V), and a PTFT (p-channel TFT) has a mobility of 120-180cm²/V.s and an S-value of 75-100 mV/dec. (V_(D)=1 V).

In particular, the S-value is extremely small, that is, less than ½ ofthe S-values of the conventional high-temperature and low-temperaturesilicon TFTs.

Embodiment 2

This embodiment is directed to a case where the gate insulating filmforming method is modified in the manufacturing method of the firstembodiment.

FIGS. 3A-3D show a manufacturing method according to this embodiment. Acrystalline silicon film 208 having a lateral growth region is formed bythe steps of FIGS. 1A and 1B. The thickness of an amorphous silicon filmas a starting film is set at 500 Å.

Once the crystalline silicon film 208 is formed, a 200-Å-thick thermaloxidation film 209 is formed by performing a heat treatment of 950° C.for 30 minutes or more by an electrical furnace in an oxygen atmospherecontaining HCl at 3%. (FIG. 3A).

After the thermal oxidation film 209 is removed, the crystalline siliconfilm 208 is patterned into a pattern 210 which will become a TFT activelayer. (FIG. 3B).

Next, a 1,000-Å-thick gate insulating film 304 is formed by plasma CVD.(FIG. 3C).

A 300-Å-thick thermal oxidation film 211 is then formed by a heattreatment of 950° C. for 30 minutes or more by an electrical furnace inan oxygen atmosphere containing HCl at 3%. (FIG. 3D).

In this step, the thermal oxidation film grows inside the CVD oxide film304 as shown in FIG. 3D. A sum of thicknesses of the thermal oxide filmsformed is thicker than the final thickness of the crystalline siliconfilm.

When the manufacturing process of this embodiment is employed. thelaminate film of the thermal oxidation film 211 and the CVD oxide film304 constitutes a gate insulating film.

When the manufacturing process of this embodiment is employed. thedensity of boundary energy states at the boundary between the gateinsulating film and the active layer can be made low.

Embodiment 3

This embodiment is directed to a manufacturing method of an activematrix circuit region of an active matrix liquid crystal display device.

FIGS. 4A-4D show a manufacturing process according to this embodiment.First, the state of FIG. 2D (also shown in FIG. 4A) is obtained by theprocess of the first embodiment.

Next, a 2,000-Å-thick silicon nitride film 401 as a first interlayerinsulating film is formed by plasma CVD. A polyimide resin film 402 isformed thereon by spin coating, to obtain the state of FIG. 4B. Resinmaterials other than polyimide, such as polyamide, polyimideamide andacrylic resin, may also be used.

After contact holes are formed so as to reach the source region 311 andthe drain region 315, a source electrode 403 and a drain electrode 403are formed each of which is a laminate film of a titanium film, analuminum film, and a titanium film. The source electrode 403 is soformed as to extend from a source line. (FIG. 4C).

Part of the source electrode 403 serves as an electrode for forming anauxiliary capacitor.

After the formation of the source and drain electrodes 403, a polyimideresin film 404 as a second interlayer insulating film is formed. Thus,the state of FIG. 4C is obtained.

Next, after an opening is formed through the resin interlayer insulatingfilm 404, a black matrix (BM) 405 is formed which is a laminate film ofa titanium film and an aluminum film. The black matrix 405 serves as anelectrode for forming an auxiliary capacitor as well as alight-shielding film (primary function).

After the formation of the black matrix 405, a polyimide resin film 406is formed as a third interlayer insulating film. Then, after a contacthole for the drain electrode 403 is formed, an ITO pixel electrode 407is formed.

In this manner, there is obtained a structure in which the polyimideresin film 406 is interposed between pattern of the black matrix 405serving as an auxiliary capacitor and the pixel electrode 407 pattern.

Embodiment 4

This embodiment is directed to a case where the method of forming acontact to the gate electrode or a gate line extending from the gateelectrode is modified in the manufacturing method of the firstembodiment.

In the first embodiment (see FIG. 2E) and the third embodiment (see FIG.4D), both of the side face and the top face of the gate electrode arecovered with the dense anodic oxide film.

This structure is very effective in suppressing the occurrence ofhillocks and short-circuiting between wiring lines in the case offorming an electrode made of aluminum. However, this structure has aproblem that it is relatively difficult to form a contact because thedense anodic oxide film is strong.

This embodiment is intended to solve this problem. FIGS. 5A-5E show amanufacturing process according to this embodiment. The detailedmanufacturing conditions etc. for forming the components given the samereference numerals as the corresponding components in the otherembodiments are the same as in those embodiments.

First, an active layer pattern 210 of a crystalline silicon film isformed as shown in FIG. 5A. Thereafter, a thermal oxidation film 211 anda CVD oxide film 304 are laid one on another. In this embodiment, theCVD oxide film 304 is formed first and then the thermal oxidation film211 is formed.

Once the state of FIG. 5A is obtained, an aluminum film is formed and a500-Å-thick silicon nitride film is formed thereon. Then, patterning isperformed by using a resist mask 306, to obtain an aluminum pattern 305and a silicon nitride film 501 formed thereon. (FIG. 5B).

In this state, with the resist mask 306 left as it is, a porous anodicoxide film 308 is formed and subsequently a dense anodic oxide film 309is formed.

These anodic oxide films 308 and 309 are selectively formed on the sideface of the aluminum pattern 307 that will become a gate electrode. Thisis because the silicon nitride film 501 is formed on the top face of thealuminum pattern 307.

The resist mask 306 is then removed. Further, the exposed portions ofthe silicon oxide film 304 as well as parts of the thermal oxidationfilm 211 are removed.

Thus, the state of FIG. 5C is obtained. Once this state is obtained, theporous anodic oxide film 308 is removed.

Further, the silicon nitride film 501 is removed to obtain the state ofFIG. 5D. In this state, an impurity for imparting a proper conductivitytype is implanted by plasma doping. As a result, a source region 311,low-concentration impurity regions 312 and 314, a channel-forming recion313. and a drain region 315 are formed in a self-aligned manner.

After the impurity doping, laser light irradiation is performed torepair the doping damage (annealing) and activate the implantedimpurity.

Thus, the state of FIG. 5D is obtained. Next, an interlayer insulatingfilm 502 is formed. After contact holes are formed, a source electrode317, a gate lead-out electrode 503, a drain electrode 318 are formed toobtain the state of FIG. 5E.

In this step, the contact hole for the gate electrode 307 can be formedrelatively easily because no silicon oxide film exists on the top faceof the gate electrode 307.

Although in FIG. 5E the source/drain electrodes 317 and 318 and the gatelead-out electrode 503 are drawn on the same cross-section, actually thegate lead-out electrode 503 is so formed as to be connected to anextension of the gate electrode 307.

Embodiment 5

This embodiment is directed to a case where a glass substrate is used inthe manufacturing method of the first embodiment.

In this embodiment, a Corning 1737 glass substrate having a strain pointof 667° C. is used. The heating treatment for crystallization isperformed at 600° C. for 4 hours.

The heating treatment to form a thermal oxidation film is performed at640° C. by an electrical furnace in an oxygen atmosphere containing HClat 3 volume %. In this case, a resulting thermal oxidation film has athickness of 30 Å when the processing time is 2 hours. The effects ofthe thermal oxidation film are smaller than in the case of the firstembodiment (heat treatment temperature: 950° C.)

Embodiment 6

This embodiment is directed to a case where HCl is not contained in theatmosphere used in forming a thermal oxidation film in the manufacturingmethod of the first embodiment. In this case, the gettering effect ofnickel is smaller than in the case where the atmosphere contains HCl.

Embodiment 7

This embodiment is directed to a case where laser light irradiation isperformed after the formation of a thermal oxidation film in themanufacturing method of the first embodiment. This effectively improvesthe crystallinity.

Embodiment 8

This embodiment is examples of semiconductor devices using TFTs. FIGS.8A-8F show various examples of semiconductor devices.

FIG. 8A shows a semiconductor device called a portable informationterminal which can read necessary information from an internal storagedevice and display it on an active matrix liquid crystal display device2005 that is incorporated in a main body 2001, or display informationobtained by accessing it via telephone lines on the display device 2005.

An active matrix EL display device may be used instead of the liquidcrystal display device 2005. Various information processing circuits anda storage circuit are formed on the same substrate as an active matrixcircuit of the display device 2005 by using TFTs, to together constitutean integrated circuit 2006.

A camera section 2002 attached to a main body 2001 can take in necessaryinformation upon a manipulation on an operation switch 2004. An image tobe taken in by the camera section 2002 is supplied to the device throughan image receiving section 2003.

FIG. 8B shows a display device called a head-mount display. A main body2101 of this device is mounted on the head of a user. This device has afunction of displaying an image in front of the eyes several centimetersapart therefrom with two active matrix liquid crystal display devices2102. This device allows the user to see a virtual image.

FIG. 8C shows a car navigation system. This device has a function ofmeasuring the position by using a signal coming from an artificialsatellite and received by an antenna 2204. The measured position isdisplayed on an active matrix liquid crystal display device 2202.Information to be displayed is selected by using operation switches2203.

An active matrix EL display device may be used instead of the liquidcrystal display device 2202.

FIG. 8D shows a cellular telephone set. A main body 2301 is equippedwith an antenna 2306, a voice input section 2303, and a voice outputsection 2302.

A user manipulates operation switches 2305 to make a call. Various typesof image information are displayed on a display device 2304, which maybe either an active matrix liquid crystal display device or an activematrix EL display device.

FIG. 8E shows a portable video camera. This device has a function ofstoring an image that is received by an image receiving section 2406onto a magnetic tape that is incorporated in a main body 2401.

Various types of digital processing are performed on the image by anintegrated circuit 2407. The integrated circuit 2407 may be acombination of conventional IC chips or may be constituted by using TFTsaccording to the invention. As a further alternative, the integratedcircuit 2407 may be a combination of the above two types ofconfigurations.

An image received by the image receiving section 2406 or stored in theinternal magnetic tape is displayed on an active matrix liquid crystaldisplay device 2402. Manipulations on the device are performed by usingoperation switches 2404. The device is powered by a battery 2405.

FIG. 8F shows a projection display device. This device has a function ofprojecting onto a screen 2505 an image produced by a main body 2501.

The main body 2501 has a light source 2502, an active matrix liquidcrystal display device for forming an image by optically modulatinglight that is emitted from the light source 2502, and an optical system2504 for projecting the image thus formed.

In the above devices, the liquid crystal display device may be either ofa transmission type or of a reflection type except for the device ofFIG. 8B.

A 9-stage ring oscillator was constructed by combining PTFTs and NTFTsproduced according to the invention. Oscillation at more than 400 MHzwas attained in this ring oscillator.

In view of the fact that actual circuits are generally designedfrequency that is about 10% of the oscillation frequency of a ringoscillator, it is concluded that a circuit capable of operating at about40 MHz can be constructed by using the above TFTs.

As exemplified above, the invention can provide TFTs which canconstitute a circuit that is required to operate at high speed (ingeneral, at more than several tens of megahertz).

In particular, the invention provides a very small S value less than 100mV/dec.), which is equivalent to an S value of a MOS sistor formed on asingle crystal silicon wafer.

The invention allows various circuits that are required to operate athigh speed to be integrated on the same substrate by using TFTs.Further, the invention provides a manufacturing method of such aconfiguration.

What is claimed is:
 1. A portable information terminal device having anactive matrix EL display device, said display device comprising: asubstrate having an insulating surface; a plurality of pixels arrangedin a matrix form; and a plurality of thin transistors disposed at saidpixels, wherein each of said thin film transistors has a channel regioncomprising crystalline silicon and an S-value not larger than 100mV/dec.
 2. A device according to claim 1, wherein said channel regioncomprising crystalline silicon is crystallized utilizing a material foraccelerating crystallization of silicon.
 3. A device according to claim1, further comprising an interlayer insulating film comprising a siliconnitride formed over said plurality of thin film transistors.
 4. A deviceaccording to claim 1, further comprising an interlayer insulating filmcomprising a resin formed over said plurality of thin film transistors.5. A device according to claim 1, wherein said portable informationterminal device reads information from an internal storage device via atelephone line.
 6. A head-mount display device having an active matrixEL display device, said display device comprising: a substrate having aninsulating surface; a plurality of pixels arranged in a matrix form; anda plurality of thin film transistors disposed at said pixels, whereineach of said thin film transistors has a channel region comprisingcrystalline silicon and an S-value not larger than 100 mV/dec.
 7. Adevice according to claim 6, wherein said channel region comprisingcrystalline silicon is crystallized utilizing a material foraccelerating crystallization of silicon.
 8. A device according to claim6, further comprising an interlayer insulating film comprising a siliconnitride formed over said plurality of thin film transistors.
 9. A deviceaccording to claim 6, further comprising an interlayer insulating filmcomprising a resin formed over said plurality of thin film transistors.10. A device according to claim 6, wherein said head-mount displaydevice is mounted on a head of a user and displays a virtual image infront of user's eyes.
 11. A car navigation system having an activematrix EL display device, said display device comprising: a substratehaving an insulating surface; a plurality of pixels arranged in a matrixform; and a plurality of thin film transistors disposed at said pixels,wherein each of said thin film transistors has a channel regioncomprising crystalline silicon and an S-value not larger than 100mV/dec.
 12. A system according to claim 11, wherein said channel regioncomprising crystalline silicon is crystallized utilizing a material foraccelerating crystallization of silicon.
 13. A system according to claim11, further comprising an interlayer insulating film comprising asilicon nitride formed over said plurality of thin film transistors. 14.A system according to claim 11, further comprising an interlayerinsulating film comprising a resin formed over said plurality of thinfilm transistors.
 15. A system according to claim 11, wherein said carnavigation system measures a position by receiving a signal coming froman artificial satellite.
 16. A cellular telephone device having anactive matrix EL display device, said display device comprising: asubstrate having an insulating surface; a plurality of pixels arrangedin a matrix form; and a plurality of thin film transistors disposed atsaid pixels, wherein each of said thin film transistors has a channelregion comprising crystalline silicon and an S-value not larger than 100mV/dec.
 17. A device according to claim 16, wherein said channel regioncomprising crystalline silicon is crystallized utilizing a material foraccelerating crystallization of silicon.
 18. A device according to claim16, further comprising an interlayer insulating film comprising asilicon nitride formed over said plurality of thin film transistors. 19.A device according to claim 16, further comprising an interlayerinsulating film comprising a resin formed over said plurality of thinfilm transistors.
 20. A device according to claim 16, wherein saidcellular telephone device further comprises an antenna, a voice input, avoice output, and operation switches to make a call.
 21. A portablevideo camera device having an active matrix EL display device, saiddisplay device comprising: a substrate having an insulating surface; aplurality of pixels arranged in a matrix form; and a plurality of thinfilm transistors disposed at said pixels, wherein each of said thin filmtransistors has a channel region comprising crystalline silicon and anS-value not larger than 100 mV/dec.
 22. A device according to claim 21,wherein said channel region comprising crystalline silicon iscrystallized utilizing a material for accelerating crystallization ofsilicon.
 23. A device according to claim 21, further comprising aninterlayer insulating film comprising a silicon nitride formed over saidplurality of thin film transistors.
 24. A device according to claim 21,further comprising an interlayer insulating film comprising a resinformed over said plurality of thin film transistors.
 25. A deviceaccording to claim 21, wherein said portable video camera device storesan image received by an image receiving section onto a magnetic tape.26. A projection display device having an active matrix EL displaydevice, said display device comprising: a substrate having an insulatingsurface; a plurality of pixels arranged in a matrix form; and aplurality of thin film transistors disposed at said pixels, wherein eachof said thin film transistors has a channel region comprisingcrystalline silicon and an S-value not larger than 100 mV/dec.
 27. Adevice according to claim 26, wherein said channel region comprisingcrystalline silicon is crystallized utilizing a material foraccelerating crystallization of silicon.
 28. A device according to claim26, further comprising an interlayer insulating film comprising asilicon nitride formed over said plurality of thin film transistors. 29.A device according to claim 26, further comprising an interlayerinsulating film comprising a resin formed over said plurality of thinfilm transistors.
 30. A device according to claim 26, wherein saidprojection display device projects an image formed therein onto ascreen.